Production of ethanol

ABSTRACT

A flow of fermenting liquor from a fermentor is separated into a yeast concentrate flow, which is recirculated to the fermentor, and a yeast-free flow which is separated in a simple evaporator unit into a first ethanol-enriched vapor flow and a first liquid bottom flow. The first vapor flow is treated to produce the desired ethanol while a major part of the first liquid bottom flow is recirculated to the fermentor. A minor, residual part of the latter flow is fed to a stripping unit where it is separated into a second ethanol-enriched vapor flow and a second liquid bottom flow exhausted of ethanol.

This application is a continuation of application Ser. No. 123,716 filedFeb. 22, 1980, now U.S. Pat. No. 4,358,536.

THE DISCLOSURE

This invention relates to a method for producing ethanol by fermentationof a substrate containing carbohydrate in one or a number of fermentors,a flow of fermenting liquor being separated for instance by centrifugalseparation into at least one yeast concentrate flow and one yeast-freeflow, the yeast concentrate flow being recirculated to the fermentorwhile the yeast-free flow is separated by distillation into one flowenriched in ethanol and one residual flow, part of which is recirculatedto the fermentor.

Such a method is disclosed in Swedish Patent Specification No.7801133-5. When separating the yeast-free flow by distillation, thebottom flow discharged from the plant is called "slop". It is essentialthat this slop be concentrated in a dissolved, solid substance,primarily of a non-fermentable nature, in order that it can be disposedof in an economical way, as by drying or burning. The slop is moreutilizable as a fertilizer in farming at a high concentration, as thecosts for transportation will be lower. In order to achieve goodoperational economy when producing ethanol, simultaneously obtainingslop, a considerable fraction of said residual flow must be recirculatedto the fermentor, and the yeast-free flow which is separated bydistillation must contain a relatively high content of dissolved, solidsubstance, primarily of a non-fermentable nature. In this way someproblems arise, however. A high concentration of dissolved, solidsubstance in the flow which is fed to a distillation plant may result inthe formation of deposits, especially in those parts of this plant whichare close to the feed point. A large part of recirculated residual flowalso means that the flow from which ethanol is to be obtained iscorrespondingly large, so that the equipment must be dimensionedaccordingly with corresponding high investment costs.

The principal object of this invention is to avoid these drawbacks in amethod of the aforementioned type which is operated in such a way thatthe content of solid substance in said residual flow is relatively high.This is achieved by feeding the yeast-free flow to a simple evaporatorunit, corresponding to one or only a few distilling stages, wherein itis separated into (1) a first vapor flow, enriched in ethanol, which isfed to a plant for the production of the desired ethanol quality, and(2) a first liquid bottom flow, of which a major part is recirculated tothe fermentor while a minor, residual part is fed to a stripping unit.In the latter, this minor part of the first liquid bottom flow isseparated into (a) a second vapor flow enriched in ethanol and (b) asecond liquid bottom flow free of ethanol. Said first liquid bottom flowcorresponds to said residual flow.

By using the new method, only a small, simple evaporator unit of thedistillation plant is exposed to the deposit-forming flow. Thisevaporator unit can be doubled without too large an investment cost, inorder that no stopping of the operation is needed for the maintenance ofthe evaporator unit (i.e., the removal of any deposits).

As a substantial part of the incoming yeast-free flow is recirculated tothe fermentor in the form of a first, liquid bottom flow after just onepassage through the evaporator unit, but before reaching the strippingunit, the latter can be designed for a substantially smaller flow thanwould have been the case if the total yeast-free flow had been separatedinto one vapor flow enriched in ehtanol and one final slop flow, free ofethanol, in one single stripping unit. By using a simple evaporator unitand a stripping unit, there is provided a slop with relatively highconcentration without the need of a corresponding high concentration ofdry solids in the fermenting liquor.

The difference in dry solids concentration between the fermenting liquorand the slop may be varied by feeding heat energy to the stripping unitaccordingly, and feeding the second vapor flow enriched in ethanol tothe simple evaporator unit as direct steam. It is important that thisdifference be maximal, as the dry solids to a large extent comprisecomponents, which increase the osmotic pressure, which in turninfluences the growing and action power of the yeast microbesextensively.

In principle, both the first and the second vapor flows enriched inethanol are fed to a plant for the production of the desired ethanolquality. This can be performed in a variety of different ways. It mustbe observed that the major part of the ethanol quantity is, of course,to be found in said first vapor flow, while the second vapor flowcontains certain amounts of ethanol but shows, above all, a highenthalpy. The ways of feeding said two vapor flows enriched in ethanolto a plant for the production of ethanol depend especially on the methodchosen for the feeding of the energy needed for the separation inquestion; that is, where this energy is fed and in which form, i.e.,indirectly by heat exchange from a suitable heating medium like steam orhot liquid, or directly in the form of steam. The use of direct steamhas the advantage that the problem with deposit formation on the heattransfer surfaces is avoided.

According to one embodiment of the invention, the second vapor flowenriched in ethanol is fed to the simple evaporator unit. This mode ofoperation will mean a simple design. Alternatively, at least part of thesecond vapor flow enriched in ethanol can be fed directly or indirectlyto the simple evaporator unit.

According to a further embodiment of the new method, at least part ofthe second vapor flow enriched in ethanol is combined with the firstvapor flow enriched in ethanol, the combined vapor flow being fed to theplant for production of the desired ethanol quality. This feeding can beperformed in diffeent ways, as by direct feeding in vapor form into arectifying column or, after condensing, by heat exchange against someflow. It may be suitable to use at least part of the available enthalpyin the first and second vapor flows enriched in ethanol for bottomheating of a rectifying column, from which there is discharged aconcentrated ethanol flow. It is also possible to use at least part ofthe second vapor flow enriched in ethanol as heating medium in arectifying column. The heating of such a rectifying column can also beperformed by a so-called thermo compressor in which the heat content ofsome of said vapors is utilized. Even the heating of the simpleevaporator unit can be performed, at least partly, by a thermocompressor.

According to another embodiment of the new method, the first vapor flowenriched in ethanol is fed to the lowest stage in a rectifying columnfor production of the desired ethanol quality. The liquid flow from thisstage is recirculated to the evaporator unit, whereby an appreciableenergy gain is obtained.

The stripping unit can be designed as a simple unit provided with only afew distilling stages, but preferably it is designed as a strippingcolumn with distillation stages enough for making it possible todischarge from its bottom a flow which is said second liquid bottom flowcontaining a low content of ethanol, of the order below 0.5% (weight).

When it is aimed at achieving a maximal difference in dry solidsconcentration between the fermenting liquor and the slop, energy is fedindirectly to the stripping unit. If the vapor flow in the strippingunit is about the same as the vapor flow in the simple evaporator unit,the difference in dry solids concentration is of major interest. Toachieve a larger difference in the ethanol concentration of liquid flowsfed to and discharged from the simple evaporator unit, the number ofdistilling stages can be increased somewhat, in order to decrease theflows between the fermentor and the simple evaporator unit, which wouldreduce the apparatus load in these flows. Thus the aim would, per se,not be to remove the ethanol extensively. In a case like this it wouldbe possible to combine the simple evaporator unit and the stripping unitto one column unit. In such case, the liquid flow returned to thefermentor would have to be discharged at a suitable level in the column.

Alternatively, the stripping unit can comprise a series of evaporatorstages, the vapor flow from such an evaporator stage (n+1) being fed tothe previous evaporator stage (n), the bottom flow from which it hasreceived. Such an embodiment can be advantageous if there are severedeposit formation problems, as in this case a minor part of thestripping unit can be replaced instead of the whole of it.

Usually it is suitable to operate the simple evaporator unit and thestripping unit at a pressure close to atmospheric pressure, but incertain cases it may be advantageous to operate these units at asomewhat reduced pressure, because the temperature is thus lowered inthem, which may be advantageous in reducing the formation of deposits.

The invention will now be disclosed more in detail, reference being madeto the accompanying drawings, in which

FIG. 1 is a schematic flowsheet for the method according to SwedishPatent Application No. 7801133-5;

FIG. 2 is a schematic view of a plant for carrying out the methodaccording to the invention;

FIG. 3 is a view similar to FIG. 2 but in which the stripping unitcomprises three evaporator units coupled in series;

FIG. 4 is a schematic view of a plant for carrying out the new methodand comprising a stripping column and a plant for production of ethanol;

FIG. 5 is a schematic view showing a material balance for a mode ofoperating the new method;

FIG. 6 is a schematic view of a plant generally similar to that in FIG.4 but including a thermo compressor for compressing vapors fed to therectifying column;

FIG. 7 is a schematic view generally similar to FIG. 4 but in which thedistillation column is combined with the stripping unit to one columnunit; and

FIG. 8 is a schematic view generally similar to FIG. 4 but in which aliquid flow is recirculated from the rectifying column to thedistillation column.

The system shown in FIG. 1 comprises a fermentor 1, a centrifugalseparator 2 for the recirculation of yeast to the fermentor, and a plant3 for the separation of a yeast-free flow into one flow, enriched inethanol, and a residual flow which is exhaused of ethanol but whichcontains a certain amount of fermentable material. The ethanol flow isdischarged through a line 4 and the residual flow through a line 5. Theyeast recirculation is performed through two lines 6 and 7, and theyeast-free flow is fed to plant 3 through a line 8. The raw materialflow is fed to the fermentor through a line 9. Evolving CO₂ isdischarged through a line 22.

The residual flow discharged from the plant at 5 is separated into twopart-flows, one of which is recirculated to the fermentor through a line10, whereas the second is discharged from the plant as slop through aline 11. The operation of such a plant is disclosed in said SwedishPatent Specification No. 7801133-5.

The plant shown in FIG. 2 comprises a simple evaporator unit 12, astripping unit 13, and a distillation unit 14. As is obvious, the plantalso comprises one fementor 1 and one centrifugal separator 2, coupledby lines in the same way as in FIG. 1. These units are the same also inFIGS. 3-5. A first liquid bottom flow from evaporator unit 12 isdischarged through a line 15, and most of it is recirculated tofermentor 1 through line 10, while a minor part is fed to stripping unit13 via a line 16 and while a second liquid bottom flow, exhausted ofethanol, is discharged through a line 17. A first vapor flow enriched inethanol from evaporator unit 12 is conveyed via a line 18, and a secondvapor flow enriched in ethanol from stripping unit 13 is conveyed via aline 19 to the distilling unit 14. The feed point of lines 18 and 19 todistilling unit 14 is indicated only schematically. From this distillingunit a concentrated ethanol flow is discharged through a line 20, and abottom flow, consisting mainly of water more or less completely freedfrom ethanol, is discharged through a line 21. This bottom flow can beused for washing any adhering ethanol from the CO₂ -flow discharged fromthe fermentation. This is shown in FIG. 2, where the bottom flow is fedvia a line 23 to a scrubber column 24, to which the CO₂ -flow from thefermentation is fed via line 22. The scrubber flow thus obtained can befed through line 25 for diluting the substrate flow delivered throughline 9. As shown, the evaporator unit 12 consists of a short columncorresponding to one or a few distilling stages, and stripping unit 13consists of a stripping column provided with a somewhat larger number ofdistilling stages, for instance clock trays.

The plant shown in FIG. 3 is similar to that in FIG. 2 with theexception that stripping unit 13 in FIG. 3 consists of three evaporatorstages 13A, 13B and 13C, arranged in series in such a way that thebottom flow from evaporator stage 13A is fed to stage 13B, from whichthe stage 13A received a vapor flow. Similarly, stage 13C received abottom flow from stage 13B, to which is fed a vapor flow from stage 13C.A slop flow from stage 13C leaves the plant through a line 17.Evaporator stage 13C is assumed to be provided with a heat exchanger inthe form of a bottom heater 26 for indirect feed of the energy neededfor the stripping. In a plant of the type shown in FIG. 3, which can beused to particular advantage if there are severe deposit problems, thesedeposits occur primarily in the evaporator unit 12 and secondly in theevaporator stage 13C.

In FIG. 4, as in FIGS. 2 and 3, the evaporator unit 12 consists of ashort column corresponding to one or a few distilling stages, and thestripping unit 13 consists of a stripping column with a somewhat largernumber of distilling stages. The second vapor flow, enriched in ethanol,is fed through line 19 to evaporator unit 12, from which a first vaporflow enriched in ethanol is conveyed through line 18 to a partialcondenser 27 consisting of one or a number of condenser stages, from thetop of which there is discharged a minor flow of volatile impuritiesthrough a line 28, and from the bottom of which there is discharged aflow through line 29 to a combined rectifying and stripping column 30provided with a bottom heater 31 and a top cooler 32. From this column30, an ethanol flow through line 33 is obtained with a high content ofethanol, while a bottom flow with a low content of ethanol is dischargedthrough a line 34. The energy for the separation in stripping unit 13and in evaporator unit 12 can be fed exclusively through a bottom heater35 in stripping unit 13, but extra energy can also be fed in a bottomheater 36 in evaporator unit 12. It is also possible to feed extraenergy in the form of live steam in evaporator unit 12.

The partial condenser 27 can also be replaced by a total condenser, fromwhich the total flow is fed to column 30.

In FIG. 5 disclosing an example of a material balance, T means totalflow in kgs, F means fermentable material and NF means solid dissilvednon-fermentable material, primarily salts, etc. Glycerine and otherorganic impurities have not been accounted for separately. DS means thesum of F+NF, i.e., "dry solids". As is obvious, the DS of the rawmaterial fed is 42.7% (weight) including added water, the DS in thefermentor 1 is 25.8% (weight) (the yeast has not been accounted for) andin the yeast-free flow fed to the simple evaporator unit 12. In thefirst liquid bottom flow 15 from the evaporator unit 12 the DS is 27.0%(weight), and it is 35.1% (weight) in the slop discharged at 17 from theplant. 92% of the first bottom flow 15 is recirculated to the fermentor,containing 2.8% (weight) ethanol. This means that there is a relativelyhigh concentration of ethanol in the recirculated flow. The energy forseparation in the evaporator unit 12 and in the stripping unit 13 is fedpartly as indirect steam in the stripping unit and partly as indirect orlive extern steam in the evaporator unit.

FIGS. 6-8 show a few further modifications of the embodiment shown inFIG. 4, in which the vapors from the stripping unit are fed to the shortdistillation column as direct heating medium. Symbols 8-35 from FIG. 4are also used in FIGS. 6-8 for the corresponding units and lines. Thevapors passing through line 18 from the short distillation column 12 aresent to the rectifying column 41 provided with a top condensor 42 forcondensing vapors from line 43 rich in ethanol, the condensate passingto a liquid discharge flow 44 and a liquid reflux flow 45. In FIG. 6 andFIG. 7 a liquid bottom flow 46 poor in ethanol is discharged from therectifying column 41.

In the modification shown in FIG. 6, the vapors from the shortdistillation column 12 are compressed in a thermo compressor 37 to forman ethalpy enriched vapor flowing through line 38. This vapor is fed toa reboiler-condensor unit 39 and is condensed to a liquid flow 40 whichis fed to the rectifying column 41.

The modification shown in FIG. 7 consists of combining the shortdistillation column 12 and the stripping unit 13 of FIG. 4 in one column47, the stripping unit 13 corresponding to part of column 47 below line10 for recirculation of liquid to the fermentor, and the distillationcolumn 12 corresponding to part of column 47 above said line 10.

In the modification according to FIG. 8, the vapor flow through line 18from the short distillation column 12 is fed to the lowest distillingstage in the rectifying column 41 and a liquid bottom flow 48 from therectifying column 41 is recirculated to the distillation column 12.

Even higher DS-content can be operated in the fermentor. The ability ofthe yeast to stand osmotic pressure probably sets a limit. DS-contentsof 25-30% (weight) are possible.

The process can be controlled in different ways. Thus the ethanolconcentration of the first vapor flow, enriched in ethanol, can becontrolled by varying the energy feed to the simple evaporator unit orto the stripping unit. The level in the fermentor can be controlled byvarying the portion of the first liquid bottom flow recirculated to thefermentor.

We claim:
 1. In the production of ethanol from a substrate containingcarbohydrate, the method which comprises the continuous steps ofdelivering the substrate to a fermentor to provide a fermenting liquor,feeding a flow of said liquor from the fermentor to a separating unitand there separating the liquor into a yeast concentrate flow and ayeast-free flow, recirculating said yeast concentrate flow to thefermentor, feeding said yeast-free flow to a short distillation columnand there separating it into a first ethanol-enriched vapor flow and afirst liquid bottom flow, treating said first vapor flow to produceethanol of a desired quality, recirculating a major part of said firstliquid bottom flow to the fermentor, feeding a minor residual part ofsaid first liquid bottom flow to a stripping unit, and separating saidresidual part in the stripping unit into a second ethanol-enriched vaporflow and a second liquid bottom flow, exhausted of ethanol.
 2. Themethod of claim 1, comprising also feeding at least a major part of saidsecond ethanol-enriched vapor flow to said distillation column.
 3. Themethod of claim 1, comprising also combining at least part of saidsecond ethanol-enriched vapor flow with said first ethanol-enrichedvapor flow.
 4. The method of claim 1, in which said first vapor flow istreated in a rectifying column to produce said ethanol of a desiredquality, the method comprising also feeding at least part of said secondvapor flow to said rectifying column as a heating medium.
 5. The methodof claim 1, in which said first vapor flow is treated in a rectifyingcolumn to produce said ethanol of a desired quality, the methodcomprising also feeding at least part of at least one of said first andsecond vapor flows to a thermo compressor, and heating said rectifyingcolumn by said thermo compressor.
 6. The method of claim 1, in whichsaid treating of said first vapor flow includes at least part of it tothe lowest distilling stage in a rectifying column, the methodcomprising also recirculating a liquid flow from said lowest stage tothe distillation column.
 7. The method of claim 1, in which the energyfor the separation in said distillation column is supplied at leastpartly in the form of external live steam.
 8. The method of claim 1, inwhich at least part of the energy for the separation in saiddistillation column is supplied indirectly by heat exchange.
 9. Themethod of claim 8, in which external steam is used for said heatexchange.
 10. The method of claim 1, in which said stripping unitincludes a plurality of evaporator stages coupled in series, the vaporflow from one evaporator stage being fed to the previous stage while thebottom flow from said stage is fed to said one stage.
 11. The method ofclaim 1, in which at least one of said separations if performed atsubstantially atmospheric pressure.
 12. The method of claim 1, in whichat least one of said separations is performed at subatmosphericpressure.
 13. The method of claim 1 in which said distillation columnand said stripping unit are combined in a single column, said yeast-freeflow being fed to the upper portion of the single column, where thedistillation column is located, said first liquid bottom flow beingdischarged from the single column at a level located at the lowerportion of the distillation column and being divided to form said majorpart and said minor residual part, said minor residual part of saidfirst bottom flow being fed to the lower portion of said single columnwhere said stripping unit is located, said second ethanol-enriched vaporflow passing upwardly in said lower portion of the single column andleaving the stripping unit at about said level.
 14. The method of claim13 in which said second ethanol-enriched vapor flow is passed directlythrough said level and into the distillation column.